The presently disclosed embodiments relate generally to the formulation of a layer that provides overall flatness to flexible imaging members and components for use in electrostatographic apparatuses. More particularly, the embodiments pertain to a flexible electrophotographic imaging member belt prepared to include an anti-curl back coating formulated to comprise a mechanically robust copolymer binder that does have enhanced wear resistance and improved imaging member curl control.
Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper.
The flexible electrostatographic imaging members may be seamless or seamed belts. A seamed belt is usually formed by cutting a rectangular imaging member sheet from a web stock, overlapping a pair of opposite ends, and welding the overlapped ends together to form a welded seam belt. Typical electrophotographic imaging member belts that include a charge transport layer and a charge generating layer on one side of a supporting substrate layer exhibit undesirable upward curling. Thus, an anti-curl back coating is usually coated onto the opposite side of the substrate layer to render imaging member belts flatness. A typical electrographic imaging member belt has a more simple material structure. It includes only a dielectric imaging layer on one side of a supporting substrate, yet an anti-curl back coating is usually needed on the opposite side of the substrate for curl control and render desired flatness.
In electrophotography, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment moves under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
Multilayered flexible photoreceptors or imaging members have at least two layers, and may include a flexible substrate, a conductive layer, an optional undercoat layer (“charge blocking layer” or “hole blocking layer”), an optional adhesive layer, a photogenerating layer (“charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, and an optional overcoating layer in either a flexible belt form or a rigid drum configuration. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer and the charge transport layer. Enhancement of charge transporting capability across these layers provides better photoreceptor performance. Multilayered flexible photoreceptor members may include an anti-curl back coating layer on the backside of the substrate, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness.
In current organic belt photoreceptors, an anticurl back coating layer is used to balance pulling force, caused by internal tension stress/strain built-up in the charge transport layer of the photoreceptor, and eliminate the upward curling. In addition, the anti-curl back coating layer should have optically suitable transmittance, for example, good transparency, so that the photoreceptor can be erased by radiation directed from the backside of the belt during electrophotographic imaging processes. In addition, since imaging member belt is encircled around and supported by a number of belt module rollers and backer bars, the anticurl back coating layer should also be mechanically robust to provide adequate wear resistance to withstand the frictional action against these belt module support components, under a dynamic belt cyclic machine functioning condition in the field.
During the manufacturing process of flexible imaging members, the charge transport layer (CTL) is coated over the charge generation layer (CGL) by applying a CTL solution coating on top of the CGL, then subsequently drying the wet applied CTL coating at elevated temperatures of about 120° C., and finally cooling down the coated photoreceptor to the ambient room temperature of about 25° C. Due to the thermal contraction mismatch between the CTL and the substrate support, the processed photoreceptor web (with finished CTL coating obtained through drying/cooling process) spontaneous curls upwardly into a roll. For example, a photorecptor web having a 29-micrometer CTL thickness and a 3½ mil polyethylene naphthalate substrate may spontaneously curl-up into a 1½-inch roll.
Typically, the CTL in a photoreceptor device has a coefficient of thermal contraction of from about 3 to about 4 times, or approximately 3.7 times, greater than that of the flexible substrate support. As a result, the CTL has a larger dimensional shrinkage than that of the flexible substrate support after through the process of application of wet CTL coating, drying it at elevated temperature, and the eventual photoreceptor web cools down to the ambient room temperature. The exhibition of photoreceptor web curling up after the completion of CTL coating is due to the consequence of larger CTL contraction as a result of the heating/cooling cycles of the manufacturing processing step. Without being bounded by theory, the development of the upward curling may be explained by the following mechanisms: (1) while the photoreceptor web after application of wet CTL coating is dried at elevated temperature (120° C.), the solvent(s) of the CTL coating solution evaporates leaving a viscous free flowing CTL liquid where the CTL releases internal stress, and maintains its lateral dimension stability without causing the occurrence of dimensional contraction; (2) during the cool down period, the temperature falls and reaches the glass transition temperature (Tg) of the CTL at 85° C., the CTL instantaneously solidifies and adheres to the underneath CGL as it transforms from being a viscous liquid into a solid layer; (3) as the temperature drops from 85° C. down to the 25° C. room ambient, the solid CTL of the photoreceptor web laterally contracts more than the flexible substrate support due to the higher thermal coefficient of dimensional contraction than that of the substrate support. Such differential in dimensional contraction results in tension strain built-up in the CTL, which pulls the photoreceptor web upwardly to exhibit curling.
To offset the curling, an anti-curl back coating (ACBC) is applied to the backside of the flexible substrate support, opposite to the side with a CTL, and render the photoreceptor web with desired flatness. Such ACBC should have optically suitable transmittance (e.g., transparency), so that the residual voltage remaining after completion of a photoelectrical imaging process on the photoreceptor surface can be erased by radiation illumination from the back side (ACBC side) of the belt during electrophotographic imaging processes. Unfortunately, the current ACBC formulations contain bisphenol polycarbonate which has limited wear resistance. The current ACBC formulation is not ideal for withstanding the frictional interaction against the machine belt support module components during usage of the image-forming apparatus. For example, such frictional interaction may increase belt drag in the image-forming apparatus and increases the load duty on the motor. The ease of material wear-off from ACBC layer generates dust and dirt debris build up inside the machine cavity, thereby negatively impacting the quality of dynamic belt motion to cause manifestation into copy printout defects. Moreover, the exacerbation of ACBC layer wear under a normal machine electrophotographic imaging belt function condition makes the layer thinner resulting in the loss of curl control for maintaining effective imaging member flatness.
Therefore, there is a need to provide an ACBC formulation which improves physical and mechanical function and does not have the described shortfalls. There is also a need to provide improved imaging members that have mechanically robust outer ACBC layer having reduced surface contact friction and less susceptibility to scratch/wear failure to effect service life extension without creating other undesirable problems.